1. Field of the Invention
The present invention relates to piezoelectric devices and methods for manufacturing the piezoelectric devices, and more particularly, to a piezoelectric device including a bulk acoustic wave resonator using bulk acoustic waves (BAWs) and a method for manufacturing the piezoelectric device.
2. Description of the Related Art
Piezoelectric devices including bulk acoustic wave resonators (hereinafter, also referred to as “BAW resonators”) having different frequencies have been suggested.
For example, as illustrated in a cross-sectional diagram of FIG. 8, a first BAW resonator 111 and a second BAW resonator 112 are provided on a substrate 101 having air gaps 101a. The first BAW resonator 111 includes electrodes 141 and 143 formed on both main surfaces of a piezoelectric film 131. The second BAW resonator 112 includes electrodes 142 and 144 formed on both main surfaces of the piezoelectric film 131. The electrodes 141 and 143 of the first BAW resonator 111 are made of gold (Au). The electrodes 142 and 144 of the second BAW resonator 112 are made of titanium (Ti). The Ti forming the electrodes 142 and 144 of the second BAW resonator 112 etches only the electrodes 141 and 143 of the first BAW resonator 111, which are made of Au, by using an iodine-based etchant, by which Ti is not subjected to etching, and the frequency of the first BAW resonator 111 is adjusted. The Au forming the electrodes 141 and 143 of the first BAW resonator 111 etches only the electrodes 142 and 144 of the second BAW resonator 112, which are made of Ti, by using a fluorine-based etchant, by which Au is not subjected to etching, and the frequency of the second BAW resonator 112 is adjusted. (See, for example, Japanese Unexamined Patent Application Publication No. 2006-50591.)
A technique for using light mass-loading which provides a frequency difference of 0.4% in order to improve the sharpness, as well as heavy mass-loading which provides a frequency difference of 3% between a series resonator and a parallel resonator, is disclosed in P. Bradley, S. Ye, J. Kim, J. H. Kim, K. Wang, H. Ko, and Y. Xie, “A Generic 2.0×2.5 mm2 UMTS FBAR Duplexer Based on 8-pole Near-Elliptic Filters”, 2009 IEEE International Ultrasonics Symposium Proceedings.
In the case of manufacturing a piezoelectric device including three or more types of resonators having different resonant frequencies, when the method employing a combination of electrode materials and etchants as described in Japanese Unexamined Patent Application Publication No. 2006-50591 is used, two or more types of etchants are required, and it is necessary to perform etching processes a plurality of times.
When the method for providing mass-loading as described in P. Bradley, S. Ye, J. Kim, J. H. Kim, K. Wang, H. Ko, and Y. Xie, “A Generic 2.0×2.5 mm2 UMTS FBAR Duplexer Based on 8-pole Near-Elliptic Filters”, 2009 IEEE International Ultrasonics Symposium Proceedings is used, there is a need to perform two or more processes for forming mass-loading (including both heavy mass-loading and light mass-loading).